1. (a) Field of the Invention
The present invention relates to a mask for polycrystallization and a method of manufacturing a thin film transistor using polycrystallization mask.
2. (b) Description of the Related Art
In general, a liquid crystal display (“LCD”) includes two panels with electrodes and a liquid crystal layer interposed therebetween. The two panels are combined with a sealant for sealing the liquid crystal layer, which is printed around the edges of the panels. The panels are supported by spacers distributed therebetween.
This LCD displays desired images by applying electric field using the electrodes to the liquid crystal layer with dielectric anisotropy and adjusting the strength of the electric field to control the amount of light passing through the panels. In this case, thin film transistors (TFTs) are used for controlling signals transmitted to the electrodes.
The most commonly used TFTs for an LCD adapts amorphous silicon as a semiconductor layer.
An amorphous silicon TFT has mobility of about 0.5 to 1 cm2/Vsec, which is suitable for a switching element of an LCD. However, it is not sufficient for a driving circuit of a display device such as an LCD or an organic EL (electro luminescent) device.
In order to overcome such a problem, an organic EL or a polysilicon TFT LCD using a polysilicon with electron mobility of 20 to 150 cm2/Vsec as a semiconductor layer has been developed. The relatively high electron mobility polysilicon TFT enables to implement a chip in glass technique that a display panel embeds its driving circuits.
In recent years, one of the most widely used methods of forming a polysilicon thin film on a glass substrate with a low melting point is an eximer laser annealing technique. The technique irradiates light with the wavelength, which can be absorbed by amorphous silicon, from an eximer laser into a amorphous silicon layer deposited on a substrate to melt the amorphous silicon layer at 1,400° C., thereby crystallizing the amorphous silicon into polysilicon. The crystal grain has a relatively uniform size ranging about 3,000–5,000 Å, and the crystallization time is only about 30–200 nanoseconds, which does not damage the glass substrate. However, there are disadvantages that non-uniform grain boundaries decrease the uniformity for electrical characteristics between the TFTs and make it hard to adjust the microstructure of the grams.
To solve these problems, a sequential lateral solidification process capable of adjusting the distribution of the grain boundaries has been developed. The process is based on the fact that the grains of polysilicon at the boundary between a liquid phase region exposed to laser beam and a solid phase region not exposed to laser beam grow in a direction perpendicular to the boundary surface. A mask having a slit pattern is provided, and a laser beam passes through transmittance areas of the mask to completely melt amorphous silicon, thereby producing liquid phase regions arranged in a slit pattern. Thereafter, the melted amorphous silicon cools down to be crystallized, and the crystal growth starts from the boundaries of the solid phase regions not exposed to the laser beam, and proceeds in the directions perpendicular to the boundary surface. The grains stop growing when they encounter each other at the center of the liquid phase region. This process is repeated after moving the slit pattern of the mask in the direction of the grain growth, and thus the sequential lateral solidification covers the whole area. The sizes of the grains can be as much as the widths of the slit pattern.
After the sequential lateral solidification, protuberances of about 400–1,000 Å are formed on the surface of the polysilicon layer along the grain boundaries. These causes stress on the boundary surface of a gate insulating layer to be formed on the semiconductor layer. The stress in this process is found to be ten times more than that in the eximer laser annealing, and this results in degrading the characteristics of the TFTs.
In addition, a dehydrogenation process for removing hydrogen contained in the amorphous silicon is required before the crystallization. Accordingly, the manufacturing method is complicated.